Applicability of nickel ferrite anode to electrolytic reduction of metal oxides in LiCl-Li2O melt at 923Â K

The development of an oxygen-evolving inert anode is of crucial importance for the electrolytic reduction process of nuclear oxide fuels using a LiCl-Li2O salt bath. Metal oxides, which have good electrical conductivity, are potential candidates for the material of the inert anode because of their corrosion resistance to O2 gas at high temperature. In this study, the applicability of a ferrite (nickel ferrite) anode to the electrolytic reduction in LiCl-Li2O melts at 923 K was investigated. The results of cyclic voltammetry indicated that O2 gas was evolved at the ferrite anode over the potential range >2.6 V (vs Li+/Li), which agrees with the value calculated from the Gibbs free energy of formation of Li2O. The current density for the O2 gas evolution at the ferrite anode was comparable to that at the conventional platinum anode, though the overpotential was somewhat high owing to its relatively high electric resistivity. It was visually observed that the O2 gas bubbles evolved at the ferrite and platinum anodes were small and promptly moved away from the anode surface, which can contribute to electrolysis with a high anode current density. In an electrolytic reduction test of Nb2O5, metallic niobium was successfully obtained by electrolysis for 5.7 h, when the potential of the ferrite anode was quite stable. It was found that the shape and weight of the ferrite anode remained unchanged while another oxide layer with ∼1 μm thickness was formed at the surface. It is concluded that nickel ferrite is a promising inert anode material for electrolytic reduction in LiCl-Li2O melts.